This invention relates to a method for manufacturing flat glass wherein the glass is formed into a flat sheet while supported on the surface of a pool of molten metal, commonly referred to as the float process. More particularly, this invention relates to a process for operating a float forming chamber so as to reduce, or eliminate, the dripping of deposits from the interior roof of the chamber onto the ribbon of glass being formed. Since the dripping of these deposits detrimentally affects the surface flatness of the glass being formed, reducing the drippage improves the quality of the glass produced.
In a float forming process, molten glass is delivered onto a pool of molten metal and is formed into a continuous ribbon or sheet of glass as disclosed, for example, in U.S. Pat. No. 710,357 of Heal; U.S. Pat. No. 789,911 of Hitchcock; U.S. Pat. Nos. 3,083,551 and 3,220,816 of Pilkington; and U.S. Pat. No. 3,843,346 of Edge et al. The temperature of the glass falls as it progresses along the elongated float chamber, typically dropping from a temperature of about 1100.degree. C. to about 600.degree. C. at the exit end where it has cooled sufficiently to be contacted by rollers or other support means to lift the formed ribbon of glass from the molten metal. The molten metal within the float chamber is usually tin, but copper, silver, or gold have occasionally been mentioned as alternative molten metals either alone or as alloys with tin.
The glass in a float forming chamber, particularly at the hot end, has a higher oxygen potential than does the tin with which it is in contact. This results in a number of chemical reactions between the glass and the tin, which result in ionic tin entering the surface of the glass ribbon. Another result is that SO.sub.3 normally present in the glass is in part reduced to elemental sulfur which goes into solution in the tin. This sulfur then evaporates from the tin as molecular tin sulfide. This vaporized tin sulfide condenses on cooler portions of the forming chamber and, in time, can build up accumulations on the roof structure of the chamber which then fall as particles or droplets onto the ribbon of glass being formed. These materials adhere and/or embed themselves in the still soft glass ribbon, producing defects in the top surface of the glass which are visible as optical distortion, causing the glass to be downgraded or in some cases rejectable. It is believed that some of the condensed tin sulfide may be reduced to elemental tin in the reducing atmosphere normally maintained within a float glass forming chamber, and thus the particles falling onto the glass ribbon may include molten elemental tin.
Many attempts have been made in the prior art to overcome the problem of condensed materials falling onto the glass ribbon, but none has been entirely satisfactory. U.S. Pat. Nos. 4,253,860 and 4,019,885 disclose periodically cleaning deposited materials from the float chamber roof. U.S. Pat. No. 4,236,906 teaches adding a material to the float chamber atmosphere to induce sublimation of the material. U.S. Pat. No. 3,356,476 discloses cleansing of the float chamber atmosphere to remove condensables therefrom. U.S. Pat. Nos. 3,494,755 and 3,597,178 provide special condensing surfaces within the float chamber in order to collect condensable materials. U.S. Pat. No. 3,494,754 discloses a technique for suppressing volatilization of materials from the molten metal in a float forming chamber by floating a layer of carbonaceous material on the surface of the metal. Purifying the molten metal bath of a float forming chamber so as to remove impurities that could volatilize is taught in U.S. Pat. Nos. 3,317,301; 3,438,761; 3,480,420; and 3,525,601. Including additives in the molten metal bath to suppress formation of impurities is taught in U.S. Pat. Nos. 3,305,337; 3,330,635; 3,330,637; 3,337,323; 3,925,052; and 3,954,432. Despite these many proposals, drippage in float glass forming chambers remains a major cause of defects in the manufacture of float glass.
The use of copper or tin/copper alloys as the molten metal support for forming float glass is disclosed in U.S. Pat. Nos. 710,357; 3,127,261; and 3,351,447.